remove std::distance usage

[neheb]

std::distance internally runs a loop, which may or may not be optimized
away. Just use simple arithmetic.

Signed-off-by: Rosen Penev [email protected]

[gabime]

Thanks @neheb

[kelbon]

lol what

[kin4stat]

template<class InputIterator>
  constexpr typename iterator_traits<InputIterator>::difference_type
    distance(InputIterator first, InputIterator last);

Distance guarantees a substraction operator, which is the same as your "fix" in your PR, if InputIterator type meets RandomAccessIterator requirements.
iterator.operations#5

std::begin and std::end with C-array in arguments returns pointer to element, [iterator.range-4], [iterator.range-55]

RandomAccessIterator requirement can be seen here [random.access.iterators#1].

We can add and substract two pointers and pointer + intergral and get implementation defined integral result: [expr.add-2.2] [expr.add-2.3]
We can compare two pointers, and get bool result: [expr.rel-4]
We can subscript pointer with resulting type X&: [expr.sub-2]
We can add/substract and assign pointers in single expression: [expr.ass-6]

Thus, the pointer meets the RandomAccessIterator requirements, and then std::distance from it and std::begin(level_string_views) is the same as it - std::begin(level_string_views)

[kin4stat]

@gabime please roll back changes

[tt4g]

@kin4stat It would be better to open PR to revert changes.